1. Field of the Invention
This invention relates to a pneumatic tire for heavy-loaded vehicles such as trucks, buses, etc. which is improved in resistance to river wear of the tread by impeding the occurrence of river wear and avoiding reverse warping deformation of belt plies during service growth of the tire.
2. Statement of Related Art
In so-called tires for a heavy load used for transport by trucks and buses in a trend toward high-speed, long-distance service as seen in the highways of Japan as well as for long-distance, high-speed cruising in the North America market, various performance characteristics such as tread wear life, resistance to river wear, etc. during service are virtually dependent on the tire profile and characteristics of the inflated or grown tire.
In particular, general use as in the Japanese domestic market with frequent stops and goes, promotes heavy abrasive wear conditions with faster wear and accordingly, the degree of occurrence of river wear is so small that the grown configuration of the tire has little effect on tire performance. However, under high-speed, long-distance cruising conditions where the wear rate is small, movement of the tread within the contact patch that is ascribable to changes in footprint contour and tire construction has an adverse effect on irregular wear performance (e.g. river wear). For this reason, a natural inflated profile of a tread inclusive of belt and carcass plies is of importance for fundamental designing of a tire for trucks and buses, and a variety of attempts have been made.
As one example of the attempts, a pneumatic tire for a heavy load is known, which tire has a tread profile formed of a single circular arc with a single radius of curvature. Because of the single curvature form of the tread, it assumes, upon contact with the ground, such a footprint contour that the length at the tread edges in the circumferential direction of the tire is shorter than the length at the tread center, and the tire is likely to induce an initiation stage of river wear since the tire must compensate for the difference in contact length by shoulder slippage.
Further, tires having a double crown tread are also known which have a tread profile constructed of two different radii of curvature. One example of a double-crown tire is disclosed in Japanese Patent First Publication No. 54-159902 (1979), wherein circular arc sections forming tread shoulder areas have a larger radius of curvature than a circular arc section forming a tread center area, and two different circular arc sections are adjacent to each other at a tangential point. Since two kinds of circular arc sections are tangent to each other without intersecting, the tire exhibits a good straight line stability, but has a problem in that when the tire undergoes side force action during running, erosion of shoulder rib edges takes place that leads to river wear in the center area.
In order to reduce river wear and shoulder step wear, a further attempt has been made to configure the tread profile so that the boundary zones between the tread center area and shoulder areas may extend in a concave bent profile (Japanese Patent First Publication No. 5-77608(1993)). However, so-called rib punching occurs because assumedly, its one-fourth tread width point (1/4 TW point) that is spaced apart a distance of 1/4 the tread width from the tire center axis is recessed, which results in slippage and tread wear that is responsible for initiation of irregular wear.
Further with a view toward simultaneously diminishing tread center wear and shoulder wear, a tire has been proposed, wherein the shoulder areas are configured as a straight line profile (Japanese Patent First Publication No. 5-77609 (1993)). This is aimed at a low profile tire and is not suited to general tires. Moreover because of the rectilinear configuration of the shoulder areas, river wear or irregular wear is likely to occur owing to a wiping action of lateral force within the contact patch that is generated by deflection of the sidewall which force is transmitted to the tread shoulder areas as the tire rolls.
With tires for heavy-loaded vehicles such as trucks or buses, in general, they are inflated and grow from high inflation pressure, heavy load and heat build-up during running to assume a specific natural inflated profile, namely, a stable configuration owing to the balance between the tension force of the carcass and the inflated pressure.
In order to elucidate the mechanism of river wear, the inventors have investigated the correlation between river wear characteristics and change or deformation of a tread, belt and carcass profile caused by inflation, using a single crown tire as shown in FIGS. 3A and 5A, on which the design of the invention will be based, and a conventional double-crown tire as shown in FIGS. 3B and 5B for comparison purposes.
With the conventional double-crown tire wherein the crown profile before and after inflation and the cross-sectional view of the deflected tire when contacting with the ground are schematically illustrated respectively in FIG. 3B and FIG. 4B, the profile of the tire before inflation as shown in dash lines is changed to that shown in solid lines, owing to tension forces exerted on the tire by inflation, wherein the belt portion forming a tire skeleton has uneven radii and exhibits reverse warping phenomenon, and the tread crown CR, carcass ply CC and belt plies BT grow unevenly. Consequently, the tread crown profile is unstable and liable to change in configuration, so that the tire lacks wear resistance.
On the other hand, the profile of a single crown tire before and after inflation is schematically illustrated in FIG. 3A and the cross-sectional view of the deflected tire is schematically illustrated in FIG. 4A respectively. As can be seen, the single-crown tire shows even growth of the tread profile, and the tension force created by inflation is shared equally by the belt and carcass portions BT,CC.
Further entering into the details thereof, the configurational change of the double-crown tire (FIG. 3B) is such that edges of the belt plies are deformed as if they might pierce through the shoulder rib SR whereby the stress of inner pressure is transmitted to the rubber of shoulder area SH and the outer ends of the shoulder rib SR are pulled downwardly. As a result, the tread crown CR is deformed into a shoulder drop profile, wherein the 1/4 TW point, namely a point spaced apart a distance of one-fourth the overall tread width from the tread center axis CL, protrudes. This is presumed to be caused by the enlargement of the belt radius as tension and belt rigidity increase with pressure, which would interfere with the tread crown configuration when the belt radius exceeds the tread crown radius, forcing out the 1/4 TW point. Since belt and ply can no longer withstand the inner pressure in such a case, the stress of inner pressure not only causes an uneven configurational change, but the tension is transmitted to the surrounding rubber material of the tread with a lower elastic modulus, which easily deforms to cause surface distortion of the tire, undermining even growth and irregular wear resistance altogether.
Additionally, because of the concave profile of the belt portion in the tread center area and the upturned belt edges as seen in FIG. 5B, the tread thickness B' above the outermost position of the belt 3BT is thinner than the tread thickness A' at the center position of the belt 3BT, and such form of the belt resembles a deformed concave shape of the belt under load as illustrated in FIG. 4B, even though no load is applied as in FIG. 3B.
The concave, reverse warping deformation of the belt ply composite after service growth is common to conventional tires, and affects tire performance because a firm tire contact with the road is not ensured. For a firm and uniform contact of the tread, the overall shape of the inflated belt ply composite combined with tread rubber thickness above the belt edge are important factors owing to the fact that reinforcement of the tread by the belt reduces movement between the tread elements and the road within the contact patch. Therefore, belt ply rigidity that resists deformation within the contact patch is one of the major factors dominating tire performances.
Accordingly, tires prone to causing a reverse warping deformation as in FIG. 3B lack belt rigidity needed for a firm tread contact, significantly reducing the road-hugging properties of the tire needed to improve irregular wear resistance. The contact with the road is impaired in the central tread area of the concave belt ply composite since the tread rubber adhered to the belt must follow the concave contour of the belt under load as in FIG. 4B.
The construction of such a belt lacks sufficient belt resistance (to tire deflection) needed to force the tread rubber against the road for a firm tread contact. The protruding, reversely warped belt edge also causes shoulder-drop configuration of the tread profile (FIG. 3B), disrupting load distribution within the contact patch as the contact pressure concentrates on the protruding rib edge at the 1/4 TW point. Combined with the wiping action of a lateral force generated within the contact patch, a round off wear of the rib edge corner develops and leads to the initiation of river wear.
On the contrary, with the other tire (single-crown), the toroidal shape of the belt with a sufficient belt rubber thickness at the belt edge BTE ensures firm contact of the central tread and an even load distribution within the contact patch for a good road-hugging property (FIG. 4A).
Now, changes of the belt ply composite BT (for example, of three plies) and the crown portion CR will be explained with reference to FIGS. 5A, 5B.
With the conventional double-crown tire (FIG. 5B), before growth or inflation (dash lines), the tread thickness a' at the tread center axis CL from the uppermost working belt ply 3BT to the tread surface is substantially equal to the tread thickness at the outermost edge of the working belt 3BT from there to the tread surface whereas after growth (solid lines), the tread thickness B' at the outermost belt edge is smaller than the tread thickness A' at the tread center axis CL.
The belt ply composite of the double-crown tire has a reasonable belt curvature before growth, but causes reverse warping deformation during growth owing to an increase in belt curvature. By contrast, with the single-crown tire (FIG. 5A), the belt ply composite grows evenly in an equal proportion.
The reverse deformation of the belt ply composite is explained referring to FIG. 6B, which is an illustrated example of an intermediate working belt ply 2BT of the three belt construction. In the intermediate belt 2BT of the double-crown tire (FIG. 6B), steel cords embedded in the diagonal direction I'J' are shown with tension F from inner pressure being applied in the direction of the X-axis. Here, a deformation inherent in a composite material, namely, a cross-elasticity effect naturally appears. Mainly caused by the absence of reinforcing cords in the diagonal direction of G'H', shear deformation in the direction of G'H' as shown in solid lines of FIG. 6B occurs, creating an a symmetrical elongation between diagonal directions of I'J' and G'H' (I'J'&lt;&lt;G'H') under tension F.
This shear deformation tends to occur conspicuously when there is an increase of belt rigidity from tension F in the X-axis owing to the low angle belt ply and when a high-tension steel cord is adopted that is constructed of two or more layers of filaments twisted in the same direction. The iron of the steel cord preferably has a carbon content of 0.75-0.85% by weight. In particular, the deformation magnitude in the circumferential direction of the tire (X-axis), which is relative to the amount of tire growth, is to be noted. As is apparent from FIG. 6B, the growth amount c' at the tread center axis CL is significantly smaller than the growth amount d' the belt (c'&lt;&lt;d'). The upturning deformation of the belt edge as shown in FIG. 5B is therefore considered to be due to the tire growth at the belt edge in the circumferential direction exceeding that of the tire center axis CL. Thus, the growth of the belt edge is excessive as compared to the growth of the belt at the tire central axis as shown in FIG. 5B, which causes the reverse warping deformation of the belt ply composite.
It should be noted that two-dimensional top views of the belt ply construction in FIGS. 6A and 6B merely illustrate asymmetrical shear deformation of the belt plies. Therefore belt angle alteration does not correspond directly with that of the actual tire. In the actual tire, the belt ply composite such as in FIG. 6B with a belt edge diameter that is similar to a diameter of the central axis (Cf. FIG. 5B) would naturally form a cylindrical shape. The open belt ends (EPI) at both belt edges during inflation would compensate for the difference in circumference by large deformation d' and belt angle alteration that result in a flare-out shape of the belt edge causing reverse warping deformation. But, the belt composite as seen in FIG. 6A forming a curvature across the cross-section of the belt would form a toroidal shape. In contrast to the belt in FIG. 6B, the tight belt ends (EPI) of the smaller belt edge diameter (Cf. FIG. 5A) would result in minimum deformation and belt angle alteration.
Thus in view of the cause for reverse warping deformation of the belt ply composite with the conventional double-crown tire as described above (FIGS. 5B, 6B), we have found that it is essential in tire design that after growth, the deformation magnitude c of the belt ply composite at the central axis be equal to or larger than the deformation magnitude d at the belt edge (c.gtoreq.d) as is shown with the single-crown tire in FIG. 6A.
Further we have found that in the prior art tire after growth, the reverse warping deformation that forces out the belt edge, and accordingly, the 1/4 TW point, results in loss of sufficient tread rubber thickness B' at the working belt edge (B&gt;B').
Accordingly, in order to impede the loss of the tread thickness above the uppermost working belt ply, which is caused by the reverse warping deformation during tire growth, this invention is designed to raise the shoulder ribs of the tire upon molding so that the shoulder thickness b above the edge of the uppermost working belt may be thicker than the tread thickness a above the working belt ply of the tread center axis while simultaneously rectifying the protrusion of the 1/4 TW point.
A general object of this invention is therefore to provide a pneumatic tire for heavy-loaded vehicles having an improved river wear resistance.